The ANSS event ID is ak0175lzdvmb and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0175lzdvmb/executive.
2017/05/02 13:13:54 63.134 -151.115 11.3 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/05/02 13:13:54:0 63.13 -151.12 11.3 4.0 Alaska
Stations used:
AK.BARN AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DIV AK.DOT
AK.EYAK AK.FIRE AK.FYU AK.GHO AK.GLB AK.HDA AK.KLU AK.KNK
AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD
AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK
AK.SKN AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR
AT.SVW2 AV.ILSW CN.DAWY IM.IL31 IU.COLA TA.H21K TA.H23K
TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K
TA.L26K TA.L27K TA.M22K TA.M24K TA.M26K TA.M27K TA.N18K
TA.N19K TA.N25K TA.O19K TA.O22K TA.POKR TA.Q19K US.EGAK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.40e+22 dyne-cm
Mw = 4.03
Z = 15 km
Plane Strike Dip Rake
NP1 13 82 -114
NP2 265 25 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.40e+22 33 123
N 0.00e+00 23 17
P -1.40e+22 48 258
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.67e+21
Mxy -5.78e+21
Mxz -2.02e+21
Myy 9.91e+20
Myz 1.21e+22
Mzz -3.66e+21
############--
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#####----------------#########----
####------------------###########---
###--------------------#############--
###---------------------##############--
#----------------------#################
##----------------------#################-
#----------------------###################
--------- -----------###################
--------- P -----------###################
-------- ----------###################
---------------------######### #######
-------------------########## T ######
-----------------########### #####
----------------##################
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Global CMT Convention Moment Tensor:
R T P
-3.66e+21 -2.02e+21 -1.21e+22
-2.02e+21 2.67e+21 5.78e+21
-1.21e+22 5.78e+21 9.91e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170502131354/index.html
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STK = 265
DIP = 25
RAKE = -20
MW = 4.03
HS = 15.0
The NDK file is 20170502131354.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 170 45 90 3.58 0.2454
WVFGRD96 2.0 170 45 90 3.73 0.3233
WVFGRD96 3.0 95 35 5 3.71 0.2514
WVFGRD96 4.0 105 25 15 3.77 0.3079
WVFGRD96 5.0 285 20 10 3.79 0.3742
WVFGRD96 6.0 280 20 5 3.81 0.4324
WVFGRD96 7.0 280 20 5 3.82 0.4777
WVFGRD96 8.0 280 15 5 3.91 0.5086
WVFGRD96 9.0 270 20 -10 3.93 0.5472
WVFGRD96 10.0 270 20 -10 3.95 0.5778
WVFGRD96 11.0 270 20 -10 3.96 0.6006
WVFGRD96 12.0 265 25 -20 3.98 0.6173
WVFGRD96 13.0 265 25 -20 4.00 0.6291
WVFGRD96 14.0 265 25 -20 4.01 0.6355
WVFGRD96 15.0 265 25 -20 4.03 0.6370
WVFGRD96 16.0 265 25 -20 4.04 0.6338
WVFGRD96 17.0 270 25 -15 4.05 0.6277
WVFGRD96 18.0 270 20 -15 4.06 0.6185
WVFGRD96 19.0 265 20 -20 4.07 0.6076
WVFGRD96 20.0 265 20 -20 4.08 0.5944
WVFGRD96 21.0 265 20 -20 4.10 0.5791
WVFGRD96 22.0 260 20 -25 4.11 0.5628
WVFGRD96 23.0 260 20 -25 4.11 0.5462
WVFGRD96 24.0 235 15 -45 4.12 0.5290
WVFGRD96 25.0 235 15 -45 4.13 0.5116
WVFGRD96 26.0 230 10 -50 4.14 0.4936
WVFGRD96 27.0 230 10 -50 4.15 0.4756
WVFGRD96 28.0 235 10 -45 4.15 0.4570
WVFGRD96 29.0 240 10 -35 4.15 0.4384
The best solution is
WVFGRD96 15.0 265 25 -20 4.03 0.6370
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3
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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00